1. Field of the Invention
This invention relates to the production of anhydrous aluminum chloride and more particularly relates to an improved process for the production of this material from aluminum chloride hexahydrate by dehydration.
2. Description of the Prior Art
Since the discovery of the process by Hall and Heroult, nearly all aluminum has been produced by electrolysis of alumina dissolved in a molten cryolite-based bath. Despite its industrial dominance, the Hall-Heroult process has several recognized disadvantages. These deficiencies have spurred research to find alternative aluminum production processes.
Among the more attractive alternative processes is the electrolytic reduction of AlCl.sub.3. This process, which offers potential advantages over the Hall-Heroult process, comprises electrolysis of anhydrous aluminum chloride dissolved in a molten electrolyte composed of one or more halides having higher electrodecomposition potentials then aluminum chloride (e.g., alkali metal chlorides or alkaline earth metal halides). See U.S. Pat. No. 3,725,222. AlCl.sub.3 (which is fed progressively to the electrolysis cells) must contain no more than trace proportions of impurities except for compatible salts of more electropositive metals such as the alkali or alkaline earth metals. For smooth cell operation and acceptably long cell life, the near complete absence of hydrolysis products of aluminum chloride is necessary. Improved methods for producing high-purity, anhydrous aluminum chloride are being sought as a result of the continuing interest in producing aluminum by electrolysis of aluminum chloride.
Unlike many hydrated salts, AlCl.sub.3.6H.sub.2 O cannot be dehydrated by simple heating because of the hydrolysis that occurs. Heating to 180.degree. C., even in a stream of HCl, produces alumina. This thermodecomposition is represented by the following equation: EQU 2AlCl.sub.3.6H.sub.2 O.fwdarw.Al.sub.2 O.sub.3 +6HCl+9H.sub.2 O
Anhydrous aluminum chloride must therefore be produced by other procedures such as by the reaction of chlorine with either molten aluminum or with aluminum oxide (Al.sub.2 O.sub.3) in an alumina-containing material such as clay or bauxite. Methods have also been proposed to produce anhydrous aluminum chloride from aluminum chloride hexahydrate.
U.S. Pat. No. 4,264,569 teaches a method for producing anhydrous aluminum chloride which comprises heating aluminum chloride hexahydrate at 200.degree. C.-450.degree. C. until the hexahydrate is substantially decomposed and reacting the decomposed material with a chlorine containing gas at 350.degree. C.-500.degree. C. to produce gaseous anhydrous aluminum chloride. Another process, referred to in the '569 patent, comprises heating aluminum chloride hexahydrate at 100.degree.-500.degree. C. to remove water and HCl and to form a basic aluminum chloride and then heating this material at 600.degree.-900.degree. C. to produce anhydrous aluminum chloride.
Low temperature thermal decomposition of AlCl.sub.3.6H.sub.2 O to Al.sub.2 O.sub.3 in molten chloroaluminate salts of alkali metals is reported by Picard, G., et al., Bull. Soc. Chim. Fr., Vol. 9-10, Pt. 1, pp. 353-60. (1981). They report that alumina formed in situ (in the melt) is more reactive with respect to HCl than Al.sub.2 O.sub.3 produced by thermodecomposition of the hexahydrate by itself (in the absence of the melt). However, they also report that when adding aluminum chloride hexahydrate to the reactor medium, it is necessary to prevent alumina formation around the grains of the solid because the alumina so formed inhibits further chemical reaction. The temperature below which this coating is negligible was determined to be 120.degree. C. Accordingly, Picard, et al., suggest the following process for producing anhydrous AlCl.sub.3 from AlCl.sub.3.6H.sub.2 O: (1) formation of a suspension of reactive alumina at a temperature of 100.degree.-120.degree. C. in molten mixtures of alkali tetrachloroaluminates, and treatment with HCl (to release water) at temperatures of 400.degree.-500.degree. C. and recovering an alkali tetrachloroaluminate melt enriched in aluminum (III). Selection of tetrachloroaluminates used to form the reaction medium is dictated by their fusion temperatures: lower-melting mixtures such as mixtures of lithium and sodium chloroaluminates and mixtures of potassium and sodium chloroaluminates are disclosed by Picard, et al. Acidic chloroaluminates (e.g., AlCl.sub.3 /NaCl having the molar ratio, 61.4/38.6) were considered but found unsatisfactory.